Use of video camera systems for unattended recording of a scene is well known. In security applications, for example, several video cameras will be connected to a recording system that periodically records a short period of video from each camera. Typically these cameras are designed to work in an auto-exposure mode, where a device called an electronic shutter varies the integration time of each video frame to match the ambient lighting conditions. Typically, these cameras produce a continuous stream of standard video signals that contain two fields of video that are interlaced to create one frame of video. For example, the standard U.S. system, NTSC or RS-170 video, has 262.5 lines of video in one field interlaced with 262.5 lines in a second field, with the second set starting 1/60 sec. later than the first set. The video monitor or recorder used with these signals combines these fields alternately to produce a full frame of video for viewing. In addition, other video systems exist that are called progressive scan systems. In these latter systems, a full frame of video is created without interlace, typically every 1/60 sec.
In these systems, the video signals are represented by a voltage. In each field or frame, there will be a maximum voltage, representing the maximum amount of light the video system can convert into a signal, and a minimum voltage representing the minimum amount of light the system can reliably represent. In a particular scene viewed by the camera, the image of the scene is converted to a video signal voltage representative of the scene with a range of voltages in between the maximum and minimum values. For a typical scene, and a particular value of auto-exposure, there may be portions of the scene that are too dark to contain any usable information, and other portions that are too light to be of use. In a surveillance application, for example, this can result in a situation where an essential part of the image is too light, or too dark, to be visible.
Heretofore one approach to overcoming the foregoing problem involves use of a combination dual exposure video camera. Combination dual exposure video cameras are commercially available, but are not as common as standard single exposure video cameras. Combination dual exposure camera systems comprise a camera that creates two different exposures of the same optical image via an electronic shutter within the time frame of each field or frame, and means for combining both exposures into a single output field or frame. This results in a single image manufactured from the two different exposure parent images which spans a wider range of scene illumination values, but which is compressed within the voltage limits of a single video frame.
This prior art approach is illustrated schematically in FIG. 1 where a dual exposure camera P1 is viewing a vase on a windowsill while a bird flies by outside. The camera P1 takes two exposures, a long period exposure represented at P4a, and a short period exposure represented at P4b, each within the time frame of a single video field (interlaced scan) or frame (progressive scan), and combines the video signals representing those two different exposures in a processor P3 according to a pre-programmed algorithm. The resulting image is represented at P4c. Both the bird and vase are visible in the combined image P4c. However, while this image combining process converts a wider range of scene illumination values, it does so at the cost of reducing contrast from the resultant image by omitting various information contained in one or the other of the parent images. This omitted information is viewed in good faith as unimportant to the final image. However, such a judgment is not always proper or acceptable, particularly in video surveillance installations. Further, when such manufactured image fields are recorded, most of this dynamic range improvement is limited in post processing. This is because the information available in the manufactured image fields formed by combining signals no longer contains the full information present in the original pictures.